if (a >= Math.min(0.2, b) || Math.pow(x, a) <= 0.9) {
                    if (invert) {
                        fract = -(normalised ? 1 : beta(a, b));
                        invert = false;
                        fract = -ibetaSeries(a, b, x, fract, normalised, pol);
                    } else {
                        fract = ibetaSeries(a, b, x, 0, normalised, pol);
                    }
                } else {
                    // swap(a, b)
                    double tmp = a;
                    a = b;
                    b = tmp;
                    // swap(x, y)
                    tmp = x;
                    x = y;
                    y = tmp;
                    invert = !invert;
                    if (y >= 0.3) {
                        if (invert) {
                            fract = -(normalised ? 1 : beta(a, b));
                            invert = false;
                            fract = -ibetaSeries(a, b, x, fract, normalised, pol);
                        } else {
                            fract = ibetaSeries(a, b, x, 0, normalised, pol);
                        }
                    } else {

fract = ibetaSeries(a, b, x, 0, normalised, pol);
                        }
                    } else {
                        // Sidestep on a, and then use the series representation:
                        final double prefix;
                        if (normalised) {
                            prefix = 1;
                        } else {
                            prefix = risingFactorialRatio(a + b, a, 20);
                        }
                        fract = ibetaAStep(a, b, x, y, 20, normalised);
                        if (invert) {
                            fract -= normalised ? 1 : beta(a, b);
                            invert = false;
                            fract = -betaSmallBLargeASeries(a + 20, b, x, y, fract, prefix, pol, normalised);
                        } else {
                            fract = betaSmallBLargeASeries(a + 20, b, x, y, fract, prefix, pol, normalised);
                        }
                    }
                }
            } else {